This invention relates to a device and a method for measuring the extent of reflective notching in photoresists.
Photoresist compositions are well known in the art for use in the fabrication of integrated circuits. Such compositions are either negative acting, where they become insoluble in developer upon exposure to actinic (activating light) radiation, or positive acting, where they become soluble in developer upon light exposure. Positive photoresists are generally considered superior with respect to resolution and aspect ratio.
Present techniques in optical projection printing (such as step and repeat cameras) can resolve images of one micron and below in photoresists with good linewidth control when planar, low reflectivity substrates are used. However, when photoresists on substrates with surface topography are exposed, there are resist-control linewidth problems introduced by optical reflections and by resist thickness non-uniformity (bulk effects).
Reflection of light from the substrate/resist interface produces variations in the light intensity and scattering in the resist during exposure, resulting in non-uniform photoresist linewidth upon development. Light can scatter from the interface into regions of the resist where exposure was not intended, resulting in linewidth variations. The amount of scattering and reflection will typically vary from region to region resulting in linewidth non-uniformity.
To eliminate the effects of chromatic aberration in exposure equipment lenses, monochromatic or quasi-monochromatic light is commonly used in resist projection techniques. Unfortunately, due to resist/substrate interface reflections, constructive and destructive interference is particularly significant when monochromatic or quasi-monochromatic light is used for photoresist exposure. In such cases the reflected light interferes with the incident light to form standing waves within the resist. In the case of highly reflective substrate regions, the problem is exacerbated since large amplitude standing waves create thin layers of underexposed resist at the wave minima. The underexposed layers can prevent complete resist development causing edge acuity problems in the resist profile. The time required to expose the photoresist is generally an increasing function of resist thickness because of the increased total amount of light required to expose an increased amount of resist. However, because of the standing wave effect, the time of exposure also includes a harmonic component which varies between successive maximum and minimum values with the resist thickness. This effect is observed for quarter wavelengths of the incident light. If the resist thickness is non-uniform, the problem becomes more severe, resulting in variable linewidth control.
Linewidth control problems also arise from substrate topography. Any image on the wafer will cause impinging light to scatter or reflect in various uncontrolled directions, affecting the uniformity of resist development. As the topography becomes more complex with efforts to design more complex circuits, the effects of reflected light become much more critical.
Various attempts have been made in the art to reduce scattering and reflection during exposure of photoresists and thereby eliminate or minimize reflective notching. One such effort involves the addition of dyes to photoresist compositions that absorb at or near the wavelength used to expose the photoresist. Typical dyes that have been used for this purpose include the coumarin family, methyl orange and methanil yellow. Considerable improvement in linewidth control and reflective notching has been achieved in this manner.
However, there now arises a need among integrated circuit and photoresist manufacturers to measure the effectiveness of a particular photoresist composition in controlling or minimizing reflective notching in a simple and inexpensive manner.